Cemeteries are absolutely some of
the most challenging conditions within which to conduct geophysical
surveys. No one geophysical surveying instrument can detect all graves
100% of the time. Furthermore, graves may still be present even if no
obvious signs of them are detected during the geophysical surveys. That
said, and assuming the ground/site conditions allow for surveys to be
done, multi-instrument geophysical surveys are the best approach for
non-destructively identifying the presence of unmarked graves in
cemeteries of any age. Three instruments that we have used in the past
have proven useful for detecting graves in a variety of settings:

Magnetometer

Magnetometers
are very sensitive to iron-based materials, including
iron objects and iron in the soil. In cemeteries the primary thing
of interest that magnetometers can detect is the disturbed soil in
grave shafts. It will not detect buried human remains or coffins
unless the coffins are buried in a brick or reinforced concrete
vault or the coffins are made from cast iron or some other magnetic
material (that does not include copper or brass). They will not
detect the hardware on coffins because in most cases it is made of
brass or is buried too deeply to be detected. In older cemeteries,
where graves have subsided as the wooden caskets disintegrate and
collapse, magnetometers are good at detecting the soil brought in
to fill in the depressions over the collapsed coffins. One major
downside to the use of magnetometers in cemeteries is that they are
very sensitive to iron objects, and in cemeteries there are many
sources for iron objects that can, in essence, blind the
magnetometer. Such iron sources include cast iron fences that
surround the cemetery or individual family plots, headstones with
iron components (such as pins that attach the headstone to a base
stone), iron plot markers, and iron-based grave furniture (like war
veteran stars, plastic flowers and wreaths that have thin steel
wires, etc.). Magnetometers can be used to survey over gravel or
paved drives and can be performed in the winter on frozen ground.

Electrical Resistance Meter

Like
magnetometers, electrical resistance meters are good at detecting
disturbed soils, which, in cemetery settings, include the soils in
grave shafts. Resistance meters send out a little electrical
current into the ground and then measure how easily it travels from
one place to another. Areas of more or less soil moisture are
essentially what this instrument measures. Many aspects of a grave
can alter soil moisture. Because graves are areas of disturbed
soil, moisture may be able to more easily penetrate, and accumulate
in, a grave. Such graves will have a lower resistance. Graves with
burial vaults, be they brick or concrete, will have a much higher
resistance since the vaults are a barrier to moisture. Resistance
meters cannot detect iron objects within or on top of the soil, so
they are not hindered by iron fences or iron objects around the
graves. However, soil moisture conditions are very important to
acquiring useful resistance data. Resistance surveys cannot be done
in the winter or when the ground is even partially frozen. They can
also be ineffective when the ground is very wet, as in late
winter/early spring, or very dry, as in late summer. That said,
ultimately the results of a resistance survey are dependant on the
soil types present in the survey area, the kinds of things that one
wants to find (like graves), and the soil moisture conditions at
the time of the survey. These factors can work together in many
different ways to make resistance surveys better or worse for
finding unmarked graves. Resistance data cannot be collected over
gravel or paved drives.

Ground Penetrating Radar

Ground
penetrating radar (GPR) surveys work by sending electromagnetic
waves (radio waves) into the ground and then detecting them as they
bounce off of things below ground and come back to the surface. The
strongest radar reflections occur when there are buried layers, or
objects, below ground that are very different from their
surroundings. GPR is very good at detecting buried layers of
different materials. In cemeteries, GPR tends to detect the sides
and bottom of a grave shaft, rather than the grave shaft fill. If
the coffin is still intact and contains air, the air will create a
strong radar reflection. Metal coffins, of any type, and burial
vaults will also create strong radar reflections. Thus, at first
glance GPR seems the most ideal of the instruments for locating
unmarked graves. However, it has its limitations. Soils that are
very wet and clayey tend to absorb the radar waves such that very
few bounce back, making it hard to detect anything below ground.
Soils with lots of gravel have the opposite effect—they produce so
many reflections that the GPR results can be too cluttered with
reflections to be of use. And, of all the instruments, GPR has the
most particular requirements for surface conditions—it does not
work well in tall grass or weeds or on very bumpy terrain. The GPR
must be in direct contact with ground, while the other instruments
do not have such requirements. Fortunately, most cemetery settings
have ideal ground surfaces for GPR surveys—mowed grass. GPR can be
used in the winter on frozen ground and it can be used over gravel
and paved drives. The great advantage of the GPR over the other
instruments is that it produces 3-dimensional data blocks--that is,
GPR data can be used to examine the ground at different depths. The
example included here is just one slice, at about 56-76 cm below
surface, from the radar data collected at this cemetery.

Looking for the remains of the
missing Goettge Patrol (WWII), Guadalcanal, Solomon Islands.

While there are other geophysical
instruments that could be used to locate unmarked graves in cemeteries,
the three mentioned here are commonly used and have located unmarked
graves all over the world. All three instruments are fairly quick to
use and maps of the survey results can be made in minutes.
Nevertheless, effective surveys require knowledgeable processing of the
data and interpretation of the survey results. And, in the end, it is
important to remember that the instruments detect differences in
various electromagnetic properties of the soil and objects within it,
not graves. The determination of whether or not graves have been
detected is an interpretation and the only way to verify an
interpretation of geophysical survey data is to core or dig up the
things detected. However, the use of multiple geophysical instruments
is the next best means, after excavation, for identifying the presence
of unmarked graves. One instrument’s survey results can serve to
support or refute the results of another, which helps improve the
reliability of our interpretations.

In addition to collecting the
geophysical data, it is also important to make an accurate map of the
cemetery, at least in the area being geophysically surveyed. We use a
laser transit/total station for making our maps as it is about the only
way to make fast, and highly reliable maps. Only survey-grade GPS
systems are accurate enough to make adequate maps of cemeteries for use
with geophysical survey data.